Breeding generally takes place near water sources and in colonies, and starts with males competing with each other using animated sexual displays. If a male is successful in finding a mate, the pair will produce one brood for the season, laying between one and five eggs in a nest made from sticks that the male has collected and the female has arranged. The males will find a new mate each season. Studies have shown that there is intense sibling rivalry over food and more aggressive chicks tend to prevail, sometimes even resulting in the weakest chick dying of starvation.
This species of egret is described as a ‘partially’ migratory bird; whether a group will migrate depends on the climate and the food availability in the area. In the northern hemisphere the cattle egret tends to migrate from cooler to warmer climates in the winter, and return in the summer when temperatures increase. In Australasia, the cattle egret travels from mainland Australia to New Zealand and Tasmania between February and April, and remains there until October or November.
The only species in its genus, the cattle egret gains its common name from its habit of commonly wandering alongside herds of cattle. It is stocky in build, has a slightly hunched posture and white-grey plumage. In the breeding season, the feathers on the head and back turn an orange-red and, remarkably, the irises and bill turn the same orange-red. The sexes differ in size and appearance, but only slightly; the male is marginally larger and has longer breeding plumes during the mating season. The cattle egret is generally a quiet bird; however, it can sometimes give a coarse, throaty call, especially during the mating season.
Range and Habitat Edit
The cattle egret has an incredibly large range and can be found nesting in North America, South America, Europe, Africa, Asia and Australia. Remarkably, this bird has not always been found across all these regions; it is believed to have originated in central Africa and then spread to many other parts of the world during the 19th century as human activities, such as the expansion of agriculture and the cattle industry, created suitable habitat for this species. The cattle egret is found in open, grassy areas, such as pastures, meadows, marshes, flood plains and swamps. The species has a preference for freshwater and is rarely found near marine environments.
The cattle egret nests in colonies, which are often, but not always, found around bodies of water. The colonies are usually found in woodlands near lakes or rivers, in swamps, or on small inland or coastal islands, and are sometimes shared with other wetland birds, such as herons, egrets, ibises and cormorants. The breeding season varies within South Asia. Nesting in northern India begins with the onset of monsoons in May. The breeding season in Australia is November to early January, with one brood laid per season. The North American breeding season lasts from April to October. In the Seychelles, the breeding season of the subspecies B.i. seychellarum is April to October.
The male displays in a tree in the colony, using a range of ritualised behaviours such as shaking a twig and sky-pointing (raising his bill vertically upwards), and the pair forms over three or four days. A new mate is chosen in each season and when re-nesting following nest failure. The nest is a small untidy platform of sticks in a tree or shrub constructed by both parents. Sticks are collected by the male and arranged by the female, and stick-stealing is rife. The clutch size can be anywhere from one to five eggs, although three or four is most common. The pale bluish-white eggs are oval-shaped and measure 45 mm × 53 mm (1.8 in × 2.1 in). Incubation lasts around 23 days, with both sexes sharing incubation duties. The chicks are partly covered with down at hatching, but are not capable of fending for themselves; they become capable of regulating their temperature at 9–12 days and are fully feathered in 13–21 days. They begin to leave the nest and climb around at 2 weeks, fledge at 30 days and become independent at around the 45th day.
The cattle egret engages in low levels of brood parasitism, and there are a few instances of cattle egret eggs being laid in the nests of snowy egrets and little blue herons, although these eggs seldom hatch. There is also evidence of low levels of intraspecific brood parasitism, with females laying eggs in the nests of other cattle egrets. As much as 30% extra-pair copulations have been noted.
The dominant factor in nesting mortality is starvation. Sibling rivalry can be intense, and in South Africa third and fourth chicks inevitably starve. In the dryer habitats with fewer amphibians the diet may lack sufficient vertebrate content and may cause bone abnormalities in growing chicks due to calcium deficiency. In Barbados, nests were sometimes raided by vervet monkeys, and a study in Florida reported the fish crow and black rat as other possible nest raiders. The same study attributed some nestling mortality to brown pelicans nesting in the vicinity, which accidentally, but frequently, dislodged nests or caused nestlings to fall. In Australia, Torresian crows, wedge-tailed eagles and White-Bellied Sea Eagles take eggs or young, and tick infestation and viral infections may also be causes of mortality.